1. Field of the Invention
This application relates to the art of optical devices that partition input optical signals into the output ports.
2. Description of Related Art
The publications in the journal “Photonics Technology Letters,” at vol. 18, p. 2392 (November 2006) and vol. 20, p. 1968 (December 2008), and the publication in the journal “Optics Express,” at vol. 18, p. 23891 (October 2010), describe an optical device, namely a waveguide that comprises a core and a cladding. The core is made with silicon (Si). The cladding is made with silicon dioxide (SiO2). Such a waveguide that comprises a core and a cladding may be termed herein a “Si-waveguide” or “Si-optical waveguide.”
The Si-optical waveguide may curve so as to bend an optical signal in a small radius of curvature of approximately 1 μm, because the difference in the index of refraction respectively of the core and cladding is large enough that the Si-waveguide may guide optical signals with relatively little power loss. The Si-waveguide may also have a submicron cross section structure that is small, because the Si-waveguide may be manufactured using processing techniques associated with silicon electronic devices.
An optical device may be downsized to as small as a silicon-electronic device using the Si-waveguide. An example of an application of the Si-waveguide is a power divider element. The wavelength separation element may partition an optical signal without changing the phase and wavelength of the optical signal. In general, such waveguides as the Y-branch waveguide and MMI waveguide are used as power divider elements.
PCT Laid-Open Patent Publication Nos. WO1995/012828, and WO2010/010878 describe that a 1*N coupler (N branch-line coupler) using the MMI waveguide may be tolerant of a dimensional error. Although the design of an MMI waveguide may include some dimensional errors, the differences in optical intensity at each of the output ports of the MMI waveguide may be suppressed. The wavelength separation element may be termed an MMI type 1*N coupler.
However, although a power divider element should ideally be designed so that an optical signal can be partitioned equally into each of the output ports, there tend to be differences in the optical intensity in each of the output ports in a Si-waveguide using the MMI type 1*N coupler (e.g. Si-type 1*N MMI coupler). It is difficult to design an optical circuit including the MMI type 1*N coupler, because the design of the optical circuit must take into account the differences in optical intensity among the output ports.